In a remarkable announcement, German researchers have declared that they “functionally cured” a patient of AIDS, eradicating all traces of the virus from his body. The feat was accomplished with a bone marrow transplant from a donor who had a genetic resistance to the virus, and researchers say that 20 months later they can find no trace of the virus in the patient’s blood, bone marrow, or organ tissue.

But the accomplishment shouldn’t be taken as a sign that a cure for the 33 million people living with AIDS is around the corner, researchers are hasty to add. Professor Rodolf Tauber from the [German] clinic said: “This is an interesting case for research. But to promise to millions of people infected with HIV that there is hope of a cure would not be right” [BBC News]. Reasons for this caution include the small number of potential donors with the HIV-resistant mutation, and the difficulty and expense of bone marrow transplants.

The idea for the treatment stems from the discovery in the 1990s that some prostitutes who had a very high risk of contracting the virus simply never got infected, because the HIV virus couldn’t enter their white blood cells, as is does with most people. The prostitutes owed their resistance to a mutation in the gene which makes the molecular “door handle” by which HIV gains access to cells. Called CCR5, the protein door handle was misshapen in the immune women, locking HIV out of their white blood cells. Since the discovery, it has been established that about 1% of Europeans have the same mutation, making them resistant to HIV [New Scientist].

In the current case of the AIDS “cure,” the treatment was conducted not by an HIV researcher, but by a doctor treating a 42-year-old patient who had both leukemia and AIDS. Bone marrow transplants are a standard treatment for leukemia patients who don’t respond to chemotherapy, but the doctor, Gero Hütter, varied the routine by finding a compatible bone marrow donor who also had the genetic resistance to HIV. Hütter first killed the patient’s own bone marrow and white blood cells, and then transplanted in the resistant cells. Now, 20 months later, Hütter says it’s possible the virus is hiding somewhere in the patient’s body, but he can find no trace of it.

Immunologist Andrew Sewell says experts are excited by the findings (which haven’t yet been published in a peer-reviewed journal), but explains that the treatment would never be offered to most AIDS patients. “The problem is most people with HIV live in sub-Saharan Africa and this is hugely expensive, you have to find a matched donor, and it’s a pretty severe and painful operation. So it’s going to be an option for very few people” [BBC News]. The promising outcome is being seen more as a proof of concept, he said, for gene therapy approaches that seek to alter the key CCR5 receptor in AIDS patients to lock out the virus.